Boston explosions (Warning: Graphic images)

Bombings killed three people and wounded more than 170 at the finish line of the 2013 Boston Marathon. (Warning: Graphic images)

(Credit: AP)

Investigators sift through evidence on Boylston Street just up from the finish line of the Boston Marathon in Boston. (April 18, 2013)

(Credit: AP)

Investigators comb through the post finish line area of the Boston Marathon at Boylston Street, two days after two bombs exploded just before the finish line in Boston. (April 17, 2013)

(Credit: AP )

An FBI investigator walks down a fire truck ladder with a bag from the top of a building at the corner of Boylston Street and Fairfield Street. Investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. (April 17, 2013)

Authorities investigate one of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon. (April 15, 2013)

(Credit: AP )

A Boston Marathon competitor and Boston police run from the area of an explosion near the finish line. (April 15, 2013)

(Credit: AP)

People react to an explosion at the 2013 Boston Marathon. Two explosions shattered the euphoria at the finish line, sending authorities out on the course to carry off the injured while stragglers were rerouted away from the smoking site of the blasts. (April 15, 2013)

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Rescue personnel respond to the scene near the finish line after explosions at the 2013 Boston Marathon. (April 15, 2013)

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A runner reacts near the explosions during the Boston Marathon. (April 15, 2013)

An unidentified woman is escorted by DEA agents from the main entrance to Brigham and Women's Hospital in Boston, where many wounded from two explosions near the finish line of the Boston Marathon were brought. (April 15, 2013)

(Credit: Bloomberg)

Officials from the Boston Fire Department Hazardous Materials Unit wrap up a garbage can along Commonwealth Avenue after two explosions occurred along the final stretch of the Boston Marathon on Boylston Street. (April 15, 2013)

(Credit: AP)

A runner retrieves his belongings from the sorting area after explosions at the finish line put a premature end to the Boston Marathon. (April 15, 2013)

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A woman in running clothes leaves the main entrance of Bringham and Women's Hospital in Boston, where many victims were brought after explosions near the finish line of the Boston Marathon. (April 15, 2013)

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Boston SWAT members leave the Boston Common staging area at the Boston Marathon where several explosions rocked the event, sending more than 140 people to local hospitals. (April 15, 2013)

Firefighters take postion on Boyltson Street near the finish line after two bombs exploded during the 117th Boston Marathon in Boston, Massachusetts. (April 15, 2013)

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State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon in Boston, Massachusetts. (April 15, 2013)

(Credit: White House)

President Barack Obama talks on the phone with FBI Director Robert Mueller to receive an update on the explosions that occurred in Boston, in the Oval Office. Seated with him are Lisa Monaco, Assistant to the President for Homeland Security and Counterterrorism, and Chief of Staff Denis McDonough. (April 15, 2013)

(Credit: AP)

Medical personnel work outside the medical tent in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston. (April 15, 2013)

(Credit: AP )

Boston police clear an area following an explosion near the finish line of the 2013 Boston Marathon in Boston. (April 15, 2013)

Two Boston Marathon volunteers hug each other, at right, as an injured person is loaded into an ambulance in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston. (April 15, 2013)

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A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square in Boston, Massachusetts. (April 15, 2013)